1. Title of the Invention
The present invention relates to a quartz crystal oscillator and, more particularly, to a quartz crystal oscillator whose frequency characteristics are temperature-compensated (to be referred to as a temperature-compensated quartz crystal oscillator hereinafter).
2. Description of the Related Art
A temperature-compensated quartz crystal oscillator which is compensated for a change in frequency of a quartz crystal vibrator based on a change in temperature has been widely used. In recent years, since communication requirements have become stricter year by year, a temperature-compensated quartz crystal oscillator having further stabilized frequency characteristics with respect to a change in temperature has been demanded.
FIG. 1 is a block diagram of a conventional temperature-compensated quartz crystal oscillator.
The temperature-compensated quartz crystal oscillator includes an oscillating circuit 1 and a temperature compensator 2. The oscillating circuit 1 includes a quartz crystal vibrator 3 serving as an oscillating element, and an electric circuit 4 coupled to the quartz crystal vibrator 3. The first electrode of the quartz crystal vibrator 3 is connected to the base of an oscillating transistor in the electric circuit 4. The quartz crystal vibrator 3 and the oscillating transistor together with other circuit elements such as a capacitor (not shown) constituting the electric circuit 4 provide, e.g., a Colpitts-type oscillating circuit. The quartz crystal vibrator 3 is of, e.g., an AT cut type, and vibrates in the thickness shear mode. Reference symbol V.sub.CC denotes a power source; and V.sub.O, an output terminal.
When the second electrode of the quartz crystal vibrator 3 is connected to a ground potential, the oscillating circuit 1 exhibits frequency-temperature characteristics represented by a curve .alpha. in FIG. 2. The frequency-temperature characteristic curve is a cubic curve having an inflection point near normal temperature, i.e., 25.degree. C. Such a cubic curve is mainly obtained by the characteristics of the quartz crystal vibrator 3.
The temperature compensator 2 includes a compensation voltage generating circuit 6 for generating a compensation voltage and a capacitor element 7 whose capacitance is variable in accordance with a voltage. The compensation voltage generating circuit 6 constitutes a serial/parallel network including a temperature transducer (e.g., a thermistor) and a resistor which are connected between the power source V.sub.CC and the ground potential. The compensation voltage generating circuit 6 generates a compensation voltage Vs1 corresponding to ambient temperature at an output terminal a in accordance with a resistance of the thermistor. The compensation voltage Vs1 serves to cause the oscillating circuit 1 to generate the frequency-temperature characteristics represented by a curve .beta. in FIG. 2. The frequency-temperature characteristic curve .beta. in FIG. 2 is a cubic curve obtained by substantially inverting the cubic curve of the frequency-temperature characteristics of the oscillating circuit 1. The capacitor element 7 is, e.g., a varactor. The cathode of the capacitor element 7 is connected to the output terminal a of the compensation voltage generating circuit 6, and the anode of the capacitor element 7 is grounded. A node between the compensation voltage generating circuit 6 and the varactor 7 is connected to the second electrode of the quartz crystal vibrator 3.
In the circuit with the above arrangement, a capacitance of the varactor 7 is changed on the basis of the compensation voltage Vs1 corresponding to ambient temperature. When the capacitance is changed, the frequency-temperature characteristics of the oscillating circuit 1 are compensated, and the compensated frequency-temperature characteristics represented by a curve .gamma. in FIG. 2 can be obtained. The frequency-temperature characteristics satisfy prescribed ratings.
In the temperature-compensated quartz crystal oscillator with the above arrangement, however, the theoretically inevitable problems are posed as follows. These problems will be described hereinafter with reference to FIG. 3.
FIG. 3 is an enlarged view of the curve .gamma. in FIG. 2. As is apparent from a curve .delta. obtained by connecting circles in FIG. 3, if ratings are set such that an allowable frequency deviation .DELTA.f/f falls within the range of .+-.2 ppm in a temperature range .DELTA.T1 defined by temperatures of -30.degree. C. to 70.degree. C., the compensated frequency-temperature characteristics satisfy the above ratings. However, if the ratings are set such that an allowable frequency deviation .DELTA.f/f falls within the range of .+-.1 ppm in the temperature range .DELTA.T1, the ratings are not satisfied in temperature ranges near -20.degree. C. and 60.degree. C. respectively surrounded by dotted lines.
When the ratings are not satisfied as described above, conventionally, a resistance in the compensation voltage generating circuit 6 is corrected by calculation on the basis of the obtained frequency-temperature characteristics. Thus, another compensation voltage generating circuit 6 is manufactured and it is used to replace the old compensation voltage generating circuit 6 to obtain an allowable deviation of .+-.1 ppm.
Even if a resistor has excellent characteristics, its resistance generally has an error of 1 to 2%. The thermistor has a standard resistance error and the B constant of the thermistor also changes in accordance with temperature. Therefore, even if the resistance in the compensation voltage generating circuit 6 is corrected, an error of about .+-.0.5 ppm is generated with respect to the characteristics obtained by theoretical calculation.
In consideration of the above situation, even if another compensation voltage generating circuit 6 is manufactured in the conventional temperature-compensated quartz crystal oscillator, it is practically difficult to obtain an allowable deviation of .+-.1 ppm or less because of the problems of the precision of each element. When the deviation .DELTA.f/f falls within a range of .+-.1 ppm or less, the production yield of the temperature-compensated quartz crystal oscillator is degraded, thus degrading productivity.